1. Technical Field
The present invention relates to noise cancelling devices and methods, and noise cancelling programs, and in particular, to a noise cancelling device and method, and a noise cancelling program for cancelling background noise signal mixed in an audio signal input from a microphone, a handset, and the like using an adaptive filter.
2. Background Art
The background noise signal mixed in the audio signal input from the microphone, the handset, or the like is a large problem in a narrow band voice coding device, a voice recognition device, or the like having large information compressibility. For the noise cancelling method aiming to cancel the acoustically superimposed noise component, a two-input noise cancelling method using an adaptive filter is described in “Adaptive Noise Cancelling: Principles and Applications” (B. Widrowet. Al., PROCEEDINGS OF IEEE, VOL. 63, NO. 12, 1975, pp. 1692-1716) (hereinafter referred to as non-patent document 1).
The two-input noise cancelling method is a method of generating a pseudo-noise signal corresponding to a noise signal component mixed at an audio input terminal, and subtracting the pseudo-noise signal from a received audio signal (coexisting signal of audio signal and noise signal) input to the audio input terminal to cancel the noise signal using an adaptive filter for approximating an impulse response of a path (noise path) through which the noise signal input to a reference input terminal passes to reach the audio input terminal.
A filter coefficient of the adaptive filter in the two-input noise cancelling method is updated by taking a correlation of an error signal obtained by subtracting the pseudo-noise signal from the received audio signal, and a reference signal input to the reference input terminal. For a coefficient updating algorithm of the adaptive filter, “LMS algorithm (Least-Mean-Square ALGORITHM)” and “LIM (Learning Identification Method)” described in “A learning method for system identification” (J. I. Nagumo, IEEE Transactions on Automatic Control, VOL. 12, NO. 3, 1967, pp. 282-287) (hereinafter referred to as non-patent document 2) are known.
However, in the noise cancelling method using the adaptive filter described above, the update of the coefficient of the adaptive filter may not follow the change of the reference signal and the filter coefficient of the adaptive filter may diverge depending on conditions such as the amplitude of the reference signal is too large with respect to an input range, or, the change in phase and amplitude of the reference signal are rapid, or, the noise component such as electrical noise on a circuit other than the audio signal is mixed in the reference signal.
When the filter coefficient of the adaptive filter diverges, the pseudo-noise signal is mistakenly generated, and the noise cancelling of the output signal may not be properly carried out. Furthermore, it takes time for the filter coefficient of the adaptive filter that is once diverged to again converge to a correct value. In order to avoid this, the divergence needs to be avoided by stopping the update of the coefficient when detecting the divergence of the filter coefficient of the adaptive filter.
The noise cancelling method for avoiding the divergence of coefficient of the adaptive filter includes a method of detecting divergence when a level of the reference signal exceeds a pre-set threshold value described in Japanese Laid-Open Patent Publication No. 5-341792 “Noise reduction device” (hereinafter referred to as patent document 1); a method of detecting divergence when a maximum value of an absolute value of the coefficient of the adaptive filter exceeds a pre-set threshold value described in Japanese Laid-Open Patent Publication No. 7-64573 “Active noise reduction device” (hereinafter referred to as patent document 2); and a method of detecting divergence when a filter power of the adaptive filter exceeds a pre-set threshold value described in Japanese Laid-Open Patent Publication No. 7-199963 “Active cancelling device of in-vehicle noise” (hereinafter referred to as patent document 3).    Non-patent document 1: Adaptive Noise Cancelling: Principles and Applications”; B. Widrowet. Al., PROCEEDINGS OF IEEE, VOL. 63, NO. 12, 1975, pp. 1692-1716    Non-patent document 2: A learning method for system identification; J. I. Nagumo, IEEE Transactions on Automatic Control, VOL. 12, NO. 3, 1967, pp. 282-287;    Patent document 1: Japanese Laid-Open Patent Publication No. 5-341792 (P. 6, FIG. 1)    Patent document 2: Japanese Laid-Open Patent Publication No. 7-64573 (P. 6, FIG. 1)    Patent document 3: Japanese Laid-Open Patent Publication No. 7-199963 (P. 5, FIG. 1)